Thus, I would say that you can "know you're done" until the next great thing comes along. That 'thing' will be something that changes your listening experience in some meaningful way, and it is not necessarily based in the quality of the sound. YMMV
For FPGA-based DACs, the next thing is usually a software update. Grimm Audio is working on an update that will allow their MU1 streamer and MU2 streamer/DAC to use control software other than Roon. Owners will get to experiment with Audirvana, JPLAY and others.
The best manufacturers listen to their customers and potential customers and act accordingly. The update is free (other manufacturers also provide free updates).
When I run out of money...lol. For the last 8 years, I have spent 2-3 hours every day on Audiogon and WBF learning the trade and dreaming about the ultimate sound. I was chasing a dream. Buying and selling components. I was searching for the next best thing.
About 6 years ago, I lost the majority of my eyesight due to a stroke. Music became that much more important to me. I was building a new house and designed a space for my music, but it wasn't a dedicated room. I wanted to build one more house to create a music studio, but I decided to enjoy what I have. During this time, I ended up buying a full Aries Cerat system. I have found my endgame. I find myself occasionally checking out Audiogon and WBF now.
Interestingly, I am now thinking to the future. Not to be morbid, and I am in pretty good health for a 71-year-old, but what happens if I were to die? I do not want to leave my wife with having to try and sell my system. She knows nothing about it. Even though I have found my endgame, I could see me making a major change next year and really simplifying my audio system. Until then I am just really enjoying my Aries Cerat system and the music from my Lucas Audio LDMS.